Final answer:
The lac repressor binds to the operator to inhibit transcription of the lac operon when lactose is absent, conserving energy. The presence of lactose induces a change that prevents the repressor from binding, allowing transcription and enabling the cell to digest lactose.
Step-by-step explanation:
LAC Operon Regulation
In the presence of lactose, the substance known as an inducer (specifically, allolactose, a derivative of lactose) binds to the lac repressor. This binding causes a conformational change in the repressor protein, which prevents it from binding to the operator sequence. With the repressor no longer blocking the operator, the RNA polymerase can bind to the promoter and initiate transcription of the lac operon, allowing the bacterium to produce the enzymes required to digest lactose.
Conversely, when lactose is absent, the inducer is not present, and the repressor maintains its ability to bind to the operator. This prevents RNA polymerase from accessing the promoter, thereby inhibiting transcription of the lac operon genes. As a result, the enzymes needed to digest lactose are not produced.
Regulation of gene expression is crucial as it conserves energy and resources, ensuring that the bacterial cell only synthesizes proteins when they are needed.